Find Inverse Laplace transform of (1)/((s^2+1)^2)

kjukks1234531

kjukks1234531

Answered question

2022-09-18

Find inverse Laplace transform of 1 ( s 2 + 1 ) 2

Answer & Explanation

Abram Jacobson

Abram Jacobson

Beginner2022-09-19Added 8 answers

Since 1 s 2 + 1 = 1 2 i ( 1 s i 1 s + i )
1 ( s 2 + 1 ) 2 = 1 4 ( 1 ( s i ) 2 + 1 ( s + i ) 2 2 s 2 + 1 ) = 1 4 [ ( s i ) 2 ( s + i ) 2 ] + 1 4 i ( 1 s i 1 s + i ) ,
which has inverse Laplace transform
1 4 [ t e i t + t e i t ] + e i t e i t 4 i = 1 2 t cos t + 1 2 sin t .
waldo7852p

waldo7852p

Beginner2022-09-20Added 2 answers

Another way is to consider:
1 ( s 2 + 1 ) 2 = 1 2 s 2 s ( s 2 + 1 ) 2
We have:
2 s ( s 2 + 1 ) 2 = 1 ( 1 ( s 2 + 1 ) )
2 s ( s 2 + 1 ) 2 = d d s { L { sin ( t ) } } = L { t sin ( t ) }
Then use Convolution theorem:
L 1 { 1 ( s 2 + 1 ) 2 } = 1 2 t sin ( t )
The Convolution integral is easier to evaluate.
f ( t ) = 1 2 0 t τ sin ( τ ) d τ
f ( t ) = 1 2 ( sin ( t ) t cos ( t ) )
L 1 { 1 ( s 2 + 1 ) 2 } = 1 2 ( sin ( t ) t cos ( t ) )

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